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SubjectRe: [PATCH v2 1/5] KVM: s390: vsie: Fix region 1 ASCE sanity shadow address checks

On 03.04.20 17:30, David Hildenbrand wrote:
> In case we have a region 1 ASCE, our shadow/g3 address can have any value.
> Unfortunately, (-1UL << 64) is undefined and triggers sometimes,
> rejecting valid shadow addresses when trying to walk our shadow table
> hierarchy.
> The result is that the prefix cannot get mapped and will loop basically
> forever trying to map it (-EAGAIN loop).
> After all, the broken check is only a sanity check, our table shadowing
> code in kvm_s390_shadow_tables() already checks these conditions, injecting
> proper translation exceptions. Turn it into a WARN_ON_ONCE().

After some testing I now triggered this warning:

[ 541.633114] ------------[ cut here ]------------
[ 541.633128] WARNING: CPU: 38 PID: 2812 at arch/s390/mm/gmap.c:799 gmap_shadow_pgt_lookup+0x98/0x1a0
[ 541.633129] Modules linked in: vhost_net vhost macvtap macvlan tap kvm xt_CHECKSUM xt_MASQUERADE nf_nat_tftp nf_conntrack_tftp xt_CT tun bridge stp llc xt_tcpudp ip6t_REJECT nf_reject_ipv6 ip6t_rpfilter ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_nat ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_nat iptable_mangle iptable_raw iptable_security nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nfnetlink ip6table_filter ip6_tables iptable_filter rpcrdma sunrpc rdma_ucm rdma_cm iw_cm ib_cm configfs mlx5_ib s390_trng ghash_s390 prng aes_s390 ib_uverbs des_s390 ib_core libdes sha3_512_s390 genwqe_card sha3_256_s390 vfio_ccw crc_itu_t vfio_mdev sha512_s390 mdev vfio_iommu_type1 sha1_s390 vfio eadm_sch zcrypt_cex4 sch_fq_codel ip_tables x_tables mlx5_core sha256_s390 sha_common pkey zcrypt rng_core autofs4
[ 541.633164] CPU: 38 PID: 2812 Comm: CPU 0/KVM Not tainted 5.6.0+ #354
[ 541.633166] Hardware name: IBM 3906 M04 704 (LPAR)
[ 541.633167] Krnl PSW : 0704d00180000000 00000014e05dc454 (gmap_shadow_pgt_lookup+0x9c/0x1a0)
[ 541.633169] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:1 PM:0 RI:0 EA:3
[ 541.633171] Krnl GPRS: 0000000000000000 0000001f00000000 0000001f487b8000 ffffffff80000000
[ 541.633172] ffffffffffffffff 000003e003defa18 000003e003defa1c 000003e003defa18
[ 541.633173] fffffffffffff000 000003e003defa18 000003e003defa28 0000001f70e06300
[ 541.633174] 0000001f43484000 00000000043ed200 000003e003def978 000003e003def920
[ 541.633203] Krnl Code: 00000014e05dc448: b9800038 ngr %r3,%r8
00000014e05dc44c: a7840014 brc 8,00000014e05dc474
#00000014e05dc450: af000000 mc 0,0
>00000014e05dc454: a728fff5 lhi %r2,-11
00000014e05dc458: a7180000 lhi %r1,0
00000014e05dc45c: b2fa0070 niai 7,0
00000014e05dc460: 4010b04a sth %r1,74(%r11)
00000014e05dc464: b9140022 lgfr %r2,%r2
[ 541.633215] Call Trace:
[ 541.633218] [<00000014e05dc454>] gmap_shadow_pgt_lookup+0x9c/0x1a0
[ 541.633257] [<000003ff804c57d6>] kvm_s390_shadow_fault+0x66/0x1e8 [kvm]
[ 541.633265] [<000003ff804c72dc>] vsie_run+0x43c/0x710 [kvm]
[ 541.633273] [<000003ff804c85ca>] kvm_s390_handle_vsie+0x632/0x750 [kvm]
[ 541.633281] [<000003ff804c123c>] kvm_s390_handle_b2+0x84/0x4e0 [kvm]
[ 541.633289] [<000003ff804b46b2>] kvm_handle_sie_intercept+0x172/0xcb8 [kvm]
[ 541.633297] [<000003ff804b18a8>] __vcpu_run+0x658/0xc90 [kvm]
[ 541.633305] [<000003ff804b2920>] kvm_arch_vcpu_ioctl_run+0x248/0x858 [kvm]
[ 541.633313] [<000003ff8049d454>] kvm_vcpu_ioctl+0x284/0x7b0 [kvm]
[ 541.633316] [<00000014e087d5ae>] ksys_ioctl+0xae/0xe8
[ 541.633318] [<00000014e087d652>] __s390x_sys_ioctl+0x2a/0x38
[ 541.633323] [<00000014e0ff02a2>] system_call+0x2a6/0x2c8
[ 541.633323] Last Breaking-Event-Address:
[ 541.633334] [<000003ff804983e0>] kvm_running_vcpu+0x3ea9ee997d8/0x3ea9ee99950 [kvm]
[ 541.633335] ---[ end trace f69b6021855ea189 ]---

Unfortunately no dump at that point in time.
I have other tests which are clearly fixed by this patch, so we should propbably go forward anyway.
Question is, is this just another bug we need to fix or is the assumption that somebody else checked
all conditions so we can warn false?

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